Component of the RNA exosome complex. Specifically part of the catalytically inactive RNA exosome core (Exo-9) complex which associates with catalytic subunits DIS3 and RRP6 in cytoplasmic- and nuclear-specific RNA exosome complex forms. Exo-9 is formed by a hexameric ring of RNase PH domain-containing subunits and peripheral S1 domain-containing components CSL4, RRP4 and RRP40 located on the top of the ring structure.